Without limiting the scope of the present invention, its background will be described with reference to explosive decompression of static seals, as an example.
A seal is a device used to close a gap or make a joint fluid-tight with respect to both liquids and gases. For example, static seals involve sealing surfaces that do not move relative to one another and dynamic seals involve sealing surfaces that do move relative to one another. In particular, static seals are zero-leakage barriers that provide a long-term complete physical barrier in a potential leakage path to which they are applied. To achieve this, the static seals must be resilient to flow and fill any irregularities in the surface being sealed while resisting extrusion into the clearance gap between the surfaces under full system pressure.
Despite their dependability and reliability, static seals in oil-field equipment can be damaged by explosive decompression when the pressure on the system containing the static seals is released. Seals usually have naturally occurring flaws that are 40 microns or less in size. Under pressure, fluid and gases can enter these voids and reach equilibrium. Moreover, as the pressure decreases, a positive internal pressure in the voids is created that increases the amount of fluid and gases that can enter these voids. If the system containing the static seals is depressurized too rapidly, the liquids and gases that enter these void sites in the static seal can cause damage which is typically exhibited by surface blisters, ruptures and fractures. Internal damage may also accompany the surface damage and occur without external evidence thereof.
The effects of explosive decompression can be minimized by slowly reducing the applied pressure to ambient. For example, the NACE Standard TM 0187-87, Evaluating Elastomeric Materials in Sour Gas Environments, recommends a decompression procedure that includes a bleed-down rate of 20 psi per minute or 100 psi and a waiting period of five minutes before continuing with another pressure reduction of 100 psi. Due to the importance of static seals, solutions to explosive decompression have not been limited to decompression procedures.
The primary structural factors that control a static seal's resistance to explosive decompression are the critical pressure, the flaw size and the shear modulus of the static seal. The critical pressure is the pressure at which the voids in the seal element expand and begin to blister or rupture. The flaw size represents the size of the naturally occurring voids in the static seal. The shear modulus, is a measure of the hardness and cross-link density, i.e., the number of connections between the polymer chains that make up the static seal, of the static seal. The critical pressure and flaw size are inherent properties of the static seal that have proven difficult to improve. Hence, existing static seals have been improved by adding reinforcing seals such as back-up rings. The reinforcing seals have shown themselves to costly and inadequate, however.
Therefore, a need has arisen for a downhole seal element that comprises a material which minimizes the effects of explosive decompression. A need has also arisen for such a downhole seal element that minimizes the critical pressure and the flaw size of the seal element. Further, a need has arisen for such a seal element that has an improved shear modulus.